1. Field of the Invention
The invention relates generally to spectrometry and particularly to hyperspectral imaging.
2. Prior Art
Hyperspectral imagers (HSI) are a class of spectrometers that record energy in many discrete spectral bands or colors simultaneously at a multitude of spatial picture elements, called pixels. Standard broadband imagers record one value at each picture element for all the detected incident energy across a wide spectrum. Hyperspectral imagers differ from standard broadband imagers by creating an additional spectral dimension in addition to the two spatial dimensions created from a two-dimensional array of detectors. Each HSI picture element may have ten to hundreds of wavelength values recorded.
Some hyperspectral imagers use a scanning approach where a scan mirror scans an image across the entrance slit of a spectrometer (pushbroom), often employing a prism or grating to disperse the various wavelengths across a detector array. Scanning hyperspectral imagers are complex and large. Such imaging spectrometers are often mounted on aircraft without a scan mirror; in such cases, the motion of the aircraft serves to scan the scene.
A class of imagers known as multispectral imagers or multispectral cameras resolve many frequencies of the image into a limited number of bands, providing a coarse measure of energy as a function of frequency. These systems use a color-sorting prism to separate the image into three spectral bands. Each prism output can drive a monochrome imaging array or a color image array. An example of this type of multispectral camera is available from Redlake MASD, Inc. (formerly DuncanTech), which has a camera configured with a 3-way color-sorting prism illuminating two monochrome imagers and one 3-color imager to cover a total of five spectral bands. The color sorting or separating prism provides bandpass filtering of the image into sub-bands. Trim filters in front of each image array can be used to provide additional wavelength selectivity and rejection of out of band energy.
One approach to achieve multispectral capability with a single imaging array, such as a complimentary metal oxide semiconductor (CMOS) or charged-coupled device (CCD) array, uses a color filter array (CFA) to limit the color band passing through to each pixel. The wide spectral response of a pixel is limited to the spectral band of the corresponding CFA element applied to that pixel. This technique is common in color video cameras and digital still cameras, using a Bayer filter pattern of interleaved red, blue, and green filters. An interpolation or de-mosaicing algorithm is used to reconstruct a color corrected image of uniform pixels from a mosaic of separate color pixels created by the filter pattern. These algorithms use a weighted combination of pixel values with neighboring pixel values. This approach creates an approximation of the hyperspectral information. Each pixel detects a different color from a slightly different position in the image scene. In a conventional pushbroom scanning system, each wavelength associated with a single spatial pixel in the cube views the exact same region of the scene. In a typical Bayer filter approach, each pixel views a slightly different region of the scene. Mathematical operations performed by image processing cannot reconstruct information lost as a result of the Bayer filter design. The de-mosaicing process produces an aesthetically acceptable result for some consumer applications. However, this loss of data can affect a variety of applications including those where a direct spectral comparison of spatially correlated images is required.
Imaging arrays fabricated with integrated circuit semiconductor technology have active and inactive areas. In addition to the light sensitive detector areas, other circuitry used for readout of the detector pixels uses area in the focal plane but does not collect light. This ratio of light sensitive and light insensitive areas is referred to as fill factor. Various technologies have different fill factors. In order to improve light sensitivity, micro-lens arrays (MLA) have been widely used in both CMOS and CCD arrays to increase the proportion of light that is directed to the light sensitive areas. In prior art designs, there is a one-to-one correspondence between a lens in the MLA and a pixel in the detector array.
A color filter array allows using a single detector array to resolve more than one color, but reconstructing the image formed from the interleaved filter array impairs accuracy for high precision hyperspectral imaging.